Conventionally, bulge processing has been known as one of the technical means of press forming, which is used for forming a tubular metal starting material into a tubular member which has a deformed cross section having expanded portions in the appropriate places across its length. The bulge process is a process for forming a tubular material into a desired form by clamping a mold in which the tubular starting material is set and then applying an internal pressure by the use of fluid pressure to the interior of the tubular starting material to allow the material to expand and fit onto the surface of the cavity of the mold. Such a conventional bulge process is usually carried out by cold forming at, for example, room temperature.
The cold bulging process, however, has a problem with its execution because it requires a very high pressure to be applied to the interior of the tubular starting material, and therefore, requires a large-scale equipment. As a result of the high pressure requirement, it is hard to process materials of high strength.
To overcome such a problem, there have been various proposed hot bulging means wherein bulging is carried out while the forming mold is heated (see Japanese Patent Application Laid-open No. 62-270229, Japanese Patent Application Laid-open No. 62-259623 and Japanese Patent Application Laid-open No. 62-259624). In these hot bulging means, both a heating function and a cooling function are provided to the mold itself, so that the starting material set in the mold is heated, swelled when a pressure is applied to its inside, then while the mold is cooled to prevent its overheating, the starting material is prevented from swelling more than necessary which prevents the mold itself from fracturing.
In conventional hot bulging means, however, the heat efficiency is poor, and deterioration in mold during the early stage of its use is caused because of the repeated heating and cooling in the same mold. Furthermore, conventional hot bulging means have a problem of taking a long time to form a product, depending on a shape of the product, and being poor in precision. Thus conventional hot bulging means are unsuitable for forming a tubular member, which is required to be of high precision and of high quality, because a sequence of forming steps are completed in one mold.